The invention relates to an energy storage device, to a system comprising an energy storage device, and to a method for actuating an energy storage device, in particular in a battery direct inverter circuit for supplying current to electric machines.
The trend is that, in the future, electronic systems which combine new energy storage technologies with electric drive technology will be used increasingly both in stationary applications, such as wind turbines or solar systems, and in vehicles, such as hybrid or electric vehicles.
The feed of polyphase current to an electric machine is generally provided by an inverter in the form of a pulse-controlled inverter. For this purpose, a DC voltage provided by a DC link can be converted into a polyphase AC voltage, for example a three-phase AC voltage. The DC link is in this case fed from a string of battery modules connected in series. In order to be able to meet the requirements for power and energy provided for a respective application, often a plurality of battery modules are connected in series in a traction battery.
A series circuit comprising a plurality of battery modules is associated with the problem that the entire string fails when a single battery module fails. Such a failure of the energy supply string can result in failure of the entire system. Furthermore, temporarily or permanently occurring power reductions of a single battery module can result in power reductions in the entire energy supply string.
The document U.S. Pat. No. 5,642,275 A1 describes a battery system comprising an integrated inverter function. Systems of this type are known under the name multilevel cascaded inverter or else battery direct inverter (BDI). Such systems comprise DC sources in a plurality of energy storage module strings, which can be connected directly to an electric machine or an electric grid. In this case, single-phase or polyphase supply voltages can be generated. The energy storage module strings in this case have a plurality of energy storage modules connected in series, wherein each energy storage module has at least one battery cell and an associated controllable coupling unit, which makes it possible to interrupt the respective energy storage module string or to bypass the respectively associated at least one battery cell or to switch the respectively associated at least one battery cell into the respective energy storage module string, depending on control signals. By suitable actuation of the coupling units, for example with the aid of pulse width modulation, suitable phase signals can also be provided for controlling the phase output voltage, with the result that it is possible to dispense with a separate pulse-controlled inverter. The pulse-controlled inverter required for controlling the phase output voltage is therefore so to speak integrated in the BDI.
BDIs generally have an increased level of efficiency and a higher degree of failsafety in comparison with conventional systems. The failsafety is ensured, inter alia, by virtue of the fact that defective, failed or incompletely effective battery cells can be disconnected from the energy supply strings by suitable bypass actuation of the coupling units. The phase output voltage of an energy storage module string can be varied by corresponding actuation of the coupling units and in particular set stepwise. The stepwise setting of the output voltage in this case results from the voltage of a single energy storage module, wherein the maximum possible phase output voltage is determined by the sum of the voltages of all of the energy storage modules in an energy storage module string.